Custom Skins for vanilla cars [Info?]

I have been in this site for a while now and only recently decided to create and account and...inspired by many of the artist in this site...decided to upload some of my work

I have a question, I have seen a lot of skins for in-game cars that already feature liveries (like the Comet Retro Custom, Sentinel Classic, Jester race car, and so on) but I was wondering if someone knows a way to create liveries for vanilla cars that don't have any kind of livery? (cars like the Turismo Classic, Furore GT, Stinger GT, so on)

I currently don't have any experience with Zmoddeler so I don't know if this is something doable, but if anyone has some sort of tutorial or some tips for a beginner, I would love to give it a try!